Process for preparing a polyether ether ketone membrane

ABSTRACT

The present invention is directed to a process for manufacturing a polyether ether ketone (PEEK) membrane which may be used for ultrafiltration or microfiltration at high temperatures. In this process for manufacturing a non-sulfonated PEEK membrane which comprises dissolving PEEK in a solvent to obtain a membrane forming stock solution, forming the stock solution into a desired shape and bringing the formed stock solution in contact with a coagulating liquid capable of coagulating PEEK, the key features involve using concentrated sulfuric acid as the solvent, dissolving PEEK therein to prepare the stock solution and keeping the stock solution at 15 degrees C or lower until initiation of membrane formation.

TECHNICAL FIELD

The present invention relates to a process for preparing a filtrationmembrane of polyether ether ketone (hereinafter referred to as PEEK).More particularly, the present invention relates to a wet, sulfuric acidprocess for preparing a non-sulfonated PEEK filtration membrane whichmay be used as an ultrafiltration or microfiltration membrane at hightemperatures because of its excellence in mechanical strength and heatand chemical resistance, and its low elution characteristics.

BACKGROUND ART

In recent years polymeric separation membranes have widely been used forthe preparation of ultrapure water for semiconductors in the electronicindustry, the filtration, purification or removal of microorganisms inthe medical, pharmaceutical or food industries, or the filtration ofindustrial waste water, etc., and the current trend is one of continuingexpansion in the range and volume of their applications and use. Forexample, a separation membrane whose ion fractions and organicsubstances are scarcely eluted and which is excellent in heat andchemical resistance is in demand for the preparation of ultrapure waterfor the production of semiconductors, while at thermal or nuclear powerplants a separation membrane excellent in heat resistance is in strongdemand for the stable, longtime filtration of condensed water havingtemperatures exceeding 100 degrees C. Thus, from the viewpoint ofmembrane performance the demand for increased heat and chemicalresistance is growing.

On the other hand, the materials which have widely been used heretoforefor manufacturing separation membranes for ultrafiltration ormicrofiltration include cellulose derivatives such as cellulose acetate,etc., polyacrylonitrile resins, polyamide resins, polymethylmethacrylate resins, polysulfone resins, polyvinylidene fluoride resins,polyolefin resins, polycarbonate resins and the like. However, becauseof highly advanced requirements for separation membranes in recent yearsas mentioned above, these materials have been unsatisfactory with regardto their elution characteristics and the heat resistance and chemicalresistance of the membranes made therefrom.

Attention is being paid to PEEK because of its excellent heatresistance, chemical resistance and low elution characteristics, so thatthis resin has been tried as a material for filtration membranes. Forexample, JP-A-61-115954 (equivalent to European patent 182506) describesa separation membrane of a sulfonated polyaryl ether ketone and aprocess for preparing the same. It is however known that such a membraneswells in water (Macromolecules, 86p, 18, 1985) and therefore degradesremarkably in mechanical strength and separation performance, forexample, in water of 60 degrees C or higher.

Further, non-sulfonated PEEK membranes have been proposed. For example,JP-A-2-136229 (equivalent to U.S. Pat. No. 4,992,485) proposes a processfor preparing a PEEK filtration membrane by using the membrane castingsolution prepared by dissolving a specifically structured PEEK in aspecific non-sulfonating acid solvent. However, the non-sulfonating acidsolvents employed in this process are strong organic acids such asmethane sulfonic acid, trichloromethane sulfuric acid, etc.,hydrofluoric acid or mixtures of them and concentrated sulfuric acid.They are not only strong in toxicity and corrosiveness but also high inprice and therefore are not suited for practical use from a commercialviewpoint. Further, these acids are disadvantageous for practical usebecause much work is required to make them harmless when they arerecovered or disposed of as wastes.

Still further, JP-A-3-56129 (equivalent to European patent 382356) andJP-A-5-192550 (equivalent to EP-A-499381) describe methods of preparingasymmetric polyetherketone resin membranes, but these methods use theaforementioned specific strong acids such as methanesulfonic acid,hydrofluoric acid, etc. as solvents for PEEK.

The acid that may be easily used industrially to dissolve PEEK isconcentrated sulfuric acid, which however has not been used in generalas a solvent for PEEK because it sulfonates the phenylene groups betweenthe ether groups of PEEK.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process forpreparing a PEEK filtration membrane which is not only substantiallynon-sulfonated by using concentrated sulfuric acid, which is easilyprocured and is inexpensive in the market instead of organic acids orhydrofluoric acid, but is also excellent in mechanical strength and heatand chemical resistance, and has desirably low elution characteristics.The PEEK membrane obtained according to the present invention is usefulas an ultrafiltration or microfiltration membrane in fields wherein heatand chemical resistance are necessary.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides

(1) in a process for preparing a non-sulfonated PEEK membrane whichcomprises dissolving PEEK in a solvent to obtain a membrane formingstock solution, forming the solution into a desired shape and contactingthe formed solution with a coagulation liquid capable of coagulatingPEEK, the improvement which comprises:

(a) using concentrated sulfuric acid as the solvent, and

(b) after dissolving PEEK in the solvent, maintaining the resultingsolution at 15 degrees C or lower until initiation of membraneformation.

(2) the process for preparing a membrane as described in (1), whereinthe concentration of concentrated sulfuric acid is 94 percent or more,

(3) the process as described in (1), wherein the membrane forming stocksolution contains a thickening agent,

(4) the process as described in (3), wherein the thickening agent ispolyvinylpyrrolidone or polyethylene glycol,

(5) the process as described in (1), wherein PEEK is dissolved in thesolvent at a temperature of 15 degrees C or lower,

(6) the process as described in (1), wherein the coagulation liquid iswater or sulfuric acid having a concentration lower than 70 percent,

(7) the process as described in (1), wherein the coagulation liquid is awater-soluble organic solvent,

(8) the process as described in (6), wherein the coagulation liquidcontains a water-soluble organic solvent,

(9) the process as described in (1), wherein after membrane formationthe membrane is subjected to heat treatment by using a heat stabilizingsolvent under a wet condition at temperatures between the glasstransition point and the melting point of PEEK,

(10) the process as described in (9), wherein the temperature of theheat treatment ranges from 150 to 320 degrees C,

(11) the process as described in (9), wherein the solubility parameterof the heat stabilizing solvent ranges from 7 to 17,

(12) the non-sulfonated PEEK membrane obtained in accordance with (1),and

(13) the non-sulfonated PEEK membrane obtained in accordance with (10)

The PEEK membrane obtained according to the process of the presentinvention retains the heat and chemical resistance that PEEK hasintrinsically in spite of using concentrated sulfuric acid which isprocurable at low prices and is widely employed in the manufacturingindustries. Further, the heat treatment for raising the crystallinity ofthe said membrane by using a heat stabilizing solvent at a wet conditionmakes it possible to obtain a heat resistant membrane durable even inwater of 130 degrees C or higher.

The membrane of the present invention is prepared in accordance with awet membrane production process. For example, in the field of polymerssuch as cellulose, etc., the process has been described by Leob andSourirajan (see Adv. Chem. Ser. 38, 117, 1963).

The process of the present invention at least comprises the followingsteps:

(A) dissolving PEEK in concentrated sulfuric acid to prepare a membraneforming stock solution,

(B) forming the solution into a desirable shape,

(C) coagulating the formed solution in a coagulation liquid, and

(D) washing to remove the concentrated sulfuric acid remaining in theresulting PEEK membrane.

First, step (A) will be described hereinafter. PEEK used in accordancewith the present invention is composed of the repeating unitsrepresented by formulas of Group 1. It may be either a homopolymercomprising the repeating units or a copolymer comprising the repeatingunits represented by two or more of these formulas. The phenylene groupsof the said repeating units may contain lower alkyl groups, halogengroups, nitro groups, nitrile groups, amino groups, phenoxyl groups,phenyl groups, biphenyl groups and substituents represented by theformulas of Group 2.

Group 1 ##STR1##

The homopolymers or copolymers comprising the repeating unitsrepresented by the formulas of Group 1 may contain as parts thereof,i.e., as copolymeric components, other repeating units, for example,those having structures as represented by the formulas of Group 3,within such quantities as may not remarkably deteriorate the inherentproperties of the said homopolymers or copolymers.

Group 3 ##STR2##

PEEK used in the present invention may be produced by knownpolymerization methods. As an example thereof, there may be mentioned amethod of condensation polymerizing an aromatic dihalogen compound and adiphenol in the presence of an alkali salt. The said polymerizationmethod is described in JP-B-57-22938, U.S. Pat. Nos. 4,113,699 and4,320,224 and JP-A-54-90296. The polymerization degree of PEEK used inthe present invention is not particularly limited. The polymerizationdegree of PEEK is expressed as reduced viscosity obtained frommeasurements at 25 degrees C with an Ostwald viscometer of a 0.1 percentPEEK solution (PEEK weight/concentrated sulfuric acid volume), thesolvent being concentrated sulfuric acid having a density of 1.83g/cm³.The measurement should be performed in the vicinity of 25 degrees Cimmediately after PEEK is dissolved in order to minimize the influenceof sulfonation. The viscometer of ca. two minute solvent efflux time isused in the measurement. The reduced viscosity of PEEK used in thepresent invention is ordinarily in the range of 0.5 to 2.5 dl/g. Inorder to obtain high mechanical strength of the resulting membrane, itis preferred that the reduced viscosity is in the range of 0.8 to 2.5dl/g.

Furthermore, the particle size of PEEK used according to the presentinvention is not especially restricted, but the smaller the better toaccelerate the dissolving rate thereof against concentrated sulfuricacid. For example, the particle size of PEEK is 5 mm or smaller,preferably 1 mm or smaller and more preferably 0.5 mm or smaller. PEEKmay be used in the form of pulverulent bodies obtained by polymerizationor smaller particles obtained by grinding pellets thereof.

According to the present invention, concentrated sulfuric acid refers tothat having a concentration of at least 85 percent or oleum. When PEEKof especially higher molecular weight is dissolved or when the PEEKconcentration is raised in a membrane forming stock solution, it ispreferred to raise the concentration of concentrated sulfuric acid to 90percent or more. When the PEEK concentration is further raised in themembrane forming stock solution or when the membrane forming stocksolution contains an additive, thickening agent or the like, theconcentration of concentrated sulfuric acid is preferably at least 94percent, particularly preferably at least 98 percent in order to raisethe dissolvability of PEEK, the additive, thickening agent and the like.According to the present invention, the concentration of sulfuric acidis expressed as percent by weight of the concentrated sulfuric acidcontent when 100 percent concentrated sulfuric acid is diluted withwater.

According to the present invention, the temperature and time required todissolve PEEK in concentrated sulfuric acid are not especiallyrestricted if the ion exchange capacity of the resulting PEEK filtrationmembrane is in the range of 0 to 0.5 meq./g. The temperature dependsupon the molecular weight of PEEK, the PEEK concentration of themembrane forming stock solution and the concentration of concentratedsulfuric acid, but should normally be within the range that may keep themembrane forming stock solution in a liquid state. The time necessary tocompletely dissolve PEEK ordinarily ranges from 3 to 100 hours.Especially, the temperature while dissolving PEEK is preferably 15degrees C or lower, more preferably 10 degrees C or lower. Further, instep (A), deaeration may be performed under vacuum in the course ofdissolving PEEK or immediately after PEEK is dissolved.

According to the present invention, an inorganic compound, lowermolecular weight organic compound or the like may be added as anadditive to the aforesaid membrane making stock solution to control thepore size of the resulting membrane. As inorganic compounds, varioussalts may be used, while as lower molecular weight organic compounds,compounds having a molecular weight of 1,000 or lower may be preferablyused. As such lower molecular weight compounds, listed arediphenylsulfone, 4,4'-dichlorodiphenylsulfone,2,4'-dichlorodiphenylsulfone, 4,4,'-difluorodiphenylsulfone,2,4'-difluorodiphenylsulfone, 2,2'-difluorodiphenylsulfone,benzophenone, 4,4'-dichlorobenzophenone, 2,4'-dichlorobenzophenone,4,4'-difluorobenzophenone, 2,4'-difluorobenzophenone,2,2'-difluorobenzophenone, 4,4'-difluoroterephthalophenone,2,4'-difluoroterephthalophenone, 4,4'-dichloroterephthalophenone,2,4'-dichloroterephthalophenone, N,N-dimethylformamide,N,N-dimethylacetoamide, dimethylsulfoxide, N-methylpyrrolidone,xanthone, terephthalic acid, isophthalic acid, salicylic acid,halogenated hydrocarbons, 1,4-butanediol, 1,3-butanediol,ethyleneglycol, diethyleneglycol, triethyleneglycol,tetraethyleneglycol, ethyleneglycol monomethylether, diethyleneglycolmonomethylether, triethyleneglycol monomethylether, tetra-ethyleneglycolmonomethylether, ethyleneglycol dimethylether, diethyleneglycoldimethylether, triethyleneglycol dimethylether, tetraethyleneglycoldimethylether, ethyleneglycol monoethylether, diethyleneglycolmonoethylether, triethyleneglycol monoethylether, tetraethyleneglycolmonomethylether, ethyleneglycol diethylether, diethyleneglycoldimethylether, triethyleneglycol diethylether, tetraethyleneglycoldimethylether, ethyleneglycol monoisopropylether, diethyleneglycolmonoisopropylether, triethyleneglycol monoisopropylether,tetraethyleneglycol monoisopropylether, propyleneglycolmonoisopropylether, ethyleneglycol diisopropylether, diethyleneglycoldiisopropylether, triethyleneglycol diisopropylether,tetraethyleneglycol diisopropylether, ethyleneglycolmonophenylether,diethyleneglycol monophenylether, triethyleneglycol monophenylether,tetraethyleneglycol monophenylether, ethyleneglycoldiphenylether,diethyleneglycol diphenylether, triethyleneglycol diphenylether,tetraethyleneglycol diphenylether, dichloroacetic acid, trichloroaceticacid, difluoroacetic acid, trifluoroacetic acid, methanesulfonic acid,chloromethanesulfonic acid, dichloromethanesulfonic acid,trichloromethanesulfonic acid, fluoromethanesulfonic acid,difluoromethanesulfonic acid, trifluoromethanesulfonic acid, glyceroland the like. It is preferred that they are uniformly dissolved in themembrane forming stock solution. They may be dispersed in a microfinestate or modified to such an extent so as not to have any bad effect onthe performance of the resulting membrane.

Furthermore, a thickening agent such as a water-soluble orwater-insoluble inorganic compound or high molecular weight organiccompound, oligomer thereof or the like may be added for the purpose ofcontrolling the viscosity of the above-mentioned membrane forming stocksolution. Examples of the inorganic compounds include utrafine silicaand the like which give a thixotropic property to the membrane formingstock solution and examples of the high molecular weight organiccompounds include polyvinylpyrrolidone, polyethyleneglycol,polyethyleneglycolmonoalkylethers, polyethyleneglycoldialkylethers,sulfonated polyetherketone, polysiloxane, polyethersulfone, polysulfone,polyetherimide, etc., mixtures thereof and oligomers thereof. Amongthem, polyvinylpyrrolidone or polyethyleneglycol is preferably used.These thickening agents may be modified in the membrane forming stocksolution to such an extent that they do not affect the performance ofthe resulting membrane.

The component ratios of the membrane forming stock solution used in theinstant invention are not especially limited if respective componentsare dissolved uniformly therein. Normally, the membrane forming stocksolution of the present invention comprises 5-20 parts by weight ofPEEK, 30-95 parts by weight of concentrated sulfuric acid, 0-20 parts byweight of an additive and 0-60 parts by weight of a thickening agent per100 parts by weight of the stock solution. The amount of said additiveis less than 40 percent by weight based on the weight of concentratedsulfuric acid used therein. The viscosity of the stock solution thusprepared according to the aforesaid component ratios and used in theinstant invention ranges from 50 to 1,000 poise at a temperature of 15degrees C.

In step (A) of the instant invention, it is preferred that agitation iscarried out in a closed system to prevent the component ratios fromchanging because of the hygroscopic property of concentrated sulfuricacid. In this case, the membrane forming stock solution is preparedunder vacuum or in the presence of a water vapor-free gas such as driednitrogen gas. On the other hand, the hygroscopicity of concentratedsulfuric acid may be controlled by adjusting the humidity in theatmosphere of the membrane forming stock solution and as a result, thismakes it possible to adjust the water content of the stock solution.

It is preferred that the state of the membrane forming stock solutionused in the present invention is one which is in the neighborhood ofmicrophase separation. If the stock solution is in such a state, theresulting filtration membrane tends to increase in water permeability.By the term "microphase separation" mentioned in the present inventionis meant a suspended state in which a polymer rich phase of the stocksolution has been separated from a polymer poor phase by the addition ofwater in extremely small increments to the prepared stock solution, withone of the phases being dispersed as microfine particles in the otherphase. Particularly, the neighborhood of the microphase separationrefers to the concentrative composition of the stock solution which mayundergo the said microphase separation when 0.01-3 parts by weight,preferably 0.05-2 parts by weight of water, is added to 100 parts byweight of the stock solution at a membrane formation temperature.Therefore, for the purpose of making the stock solution one which is inthe neighborhood of the microphase separation it is important toaccurately control the ratio of each of the components such as theconcentrated sulfuric acid, additive, thickening agent, and the like.

Following step (A), step (B) of forming the membrane forming stocksolution into a desirable shape and step (C) of coagulating the shapedmembrane forming stock solution in a coagulation liquid proceed inorder.

According to the present invention, it is necessary to cool the membraneforming stock solution immediately after step (A) and to keep thetemperature of the stock solution at 15 degrees C or lower until thebeginning of membrane formation. This temperature also refers to that ofthe piping and tanks in which the stock solution is detained or storedin the course of membrane formation and is preferably 10 degrees C orlower and more preferably 6 degrees C or lower. The temperature of thestock solution has a great influence on the sulfonation, mechanicalstrength and chemical resistance of the filtration membrane obtainedaccording to the present invention.

The membrane forming stock solution may be deaerated between steps (A)and (B). The method of deaeration is not especially restricted, but thedeaeration is carried out generally under vacuum or by means ofcentrifugal separation. In general, there is the possibility that theresulting filtration membrane will have pinholes if the deaeration stepis omitted.

According to the present invention, the form of the membrane obtained instep (B) is that of a flat-sheet, hollow fiber, tubular and capillarymembrane, though the form is not especially limited. Examples of methodsfor forming a flat-sheet membrane include application of the membraneforming stock solution to a support and the consecutive immersionthereof in a coagulation liquid, extrusion of the membrane forming stocksolution through a slit die into a coagulation liquid, and the like.Examples of the supports not attacked immediately by concentratedsulfuric acid include sheets of stainless steel, polyethylene,polypropylene, polytetrafluoroethylene, glass and the like. To obtaintubular, hollow fiber or capillary membranes, a coaxial double-tube dieis used, through an annular orifice of which a membrane forming stocksolution is extruded and through the bore of which a liquid such as agood solvent or non-solvent for the PEEK or an inert gas as a borecoagulant is passed, and the stock solution thus extruded is immersed ina coagulation liquid.

The thickness of the PEEK membrane obtained in the instant invention isnot particularly limited but ranges normally from 5 to 10,000 μm.Particularly, if the form of the membrane is that of a hollow fibermembrane, a membrane with a wall thickness ranging from 5 to 2,000 μmand an inside diameter ranging from 10 to 5,000 μm may be producedpreferably but the thickness and inside diameter are not especiallylimited.

Furthermore, according to the present invention, it is possible toobtain a composite membrane which may be produced by flow casting amembrane forming stock solution on a porous fabric substrate andcoagulating it integrally. Examples of materials for the poroussubstrates normally include fabrics of polyethylene, polypropylene,polytetrafluoroethylene, polyester, PEEK, polyether ketone (PEK),polyphenylenesulfide, carbon fiber and glass fiber.

In step (B), the temperature of the membrane forming stock solution isordinarily 80 degrees C or lower and falls within a range where thesolution is in a liquid state. For example, if the membrane formingstock solution is formed into a flat-sheet membrane or hollow fibermembrane by use of a flat slit die or a coaxial double-tube die, thetemperature of the stock solution may be adjusted by controlling thetemperature of the said dies so that it may be within theabove-mentioned temperature range. In this case, the temperature of thepiping and tanks wherein the stock solution is detained or stored is, ofcourse, 15 degrees C or lower as mentioned hereinbefore.

The coagulation liquids used in step (c), are sufficient if they arecapable of coagulating PEEK and are mixable with concentrated sulfuricacid and for example, include water, dilute sulfuric acid, acetic acid,acetic acid ester, alcohols, polyhydric alcohols, monoalkylethers ofglycols, dialkylethers of glycols, ketones, polymer-containingsolutions, and mixtures thereof. Further, inorganic salts or bases maybe added to the coagulation liquids. As the inorganic salts, preferablyused are, for example, lithium chloride, sodium chloride, calciumchloride, magnesium chloride, ammonium chloride, ammonium nitrate,lithium sulfate, magnesium perchlorate, sodium perchlorate, sodiumhypochlorite, sodium hydroxide, potassium hydroxide, sodium carbonate,potassium carbonate, potassium hydrogen carbonate and the like.

Particularly if a coagulation liquid highly capable of coagulating themembrane forming stock solution is used, a membrane having highfractionating characteristics tends to be preferably obtained. Suchcoagulation liquids are, for example, water and sulfuric acid having aconcentration of less than 70 percent.

If a coagulation liquid low in the capability of coagulating themembrane forming stock solution is used, a highly water-permeablemembrane tends to be obtained.

Coagulability may be adjusted by using a water-soluble organic solvent,a mixture of a water-soluble organic solvent and water, a mixture of astrong acid and water, a mixture of a strong acid and a water-solubleorganic solvent or the like as a coagulation liquid.

The surface of the formed stock solution may be contacted with the vaporof a non-solvent against PEEK immediately before the said solution isimmersed in the coagulation liquid.

Particularly, if the form of the membrane is that of a tube, hollowfiber or capillary tube, bore coagulants similar to the aforesaidcoagulation liquids may be used and they may be the same as or differentfrom the coagulation liquids. Various inert gases may be used as thebore coagulants.

If the temperature of the membrane forming stock solution is within arange where PEEK may not be substantially sulfonated during membraneformation, the said temperature is not especially limited but isnormally in the range of the freezing point of the stock solution to 80degrees C. If the stock solution is formed by use of a slit die orcoaxial double-tube die, the temperature of the stock solution duringmembrane formation may be controlled by heating the said die. If thetemperature of the solution is, for example, 15 degrees C or higher, itis preferred to shorten its detention time in the die to minimize thesulfonation of PEEK.

Further, the range of -10 to 90 degrees C that does not sulfonate PEEKsubstantially may be adopted as the temperature of the coagulationliquids and the bore coagulants used for forming a hollow fibermembrane.

According to the present invention, the shaped membrane forming stocksolution may be stretched when or after it is coagulated, if necessary.

Step (C) is followed by Step (D) in which residual sulfuric acid iswashed away from the resulting PEEK filtration membrane. Further, instep (D), the additive, thickening agent and coagulation liquid whichare used in the membrane forming stock solution are also removed fromthe resulting PEEK filtration membrane in addition to the removal of theresidual sulfuric acid. However, step (D) may be omitted if the residualsulfuric acid, additive and thickening agent are removed before step (D)to such an extent that problems do not occur in the heat treatmentdiscussed hereinafter or in the use of the resulting filtrationmembrane.

In step (D), the residual sulfuric acid and other components areremoved, for example, by means of the washing treatment wherein anaqueous solvent or a solvent mostly consisting of water is used as awashing solvent at temperatures within the range between 5 degrees C andthe boiling point of water. Normally, water or an alkaline water is usedpreferably. Particularly, if the form of the PEEK filtration membrane isthat of a tube, hollow fiber or capillary tube, it is also effective topass the said washing solvent through its bore.

If a small amount of sulfuric acid still remains in the said membraneafter step (D), it may be further rinsed with an organic solvent. As theorganic solvents, normally used are methanol, ethanol, propanol,acetone, methylethylketone, ethylene glycol, diethylene glycol,triethylene glycol, dimethylformamide, dimethylacetamide,N-methyl-2-pyrrolidone, etc. For example, the range of -5 to 120 degreesC may be adopted as the temperatures of the said organic solvents. Ifthickening agents or additives insoluble to aqueous solvents are used instep (A), the above-mentioned organic solvents are preferable forwashing and removing. Further, if polyvinyl pyrrolidone is used as athickening agent in step (A), residual polyvinyl pyrrolidone may bedecomposed, removed and rinsed with an aqueous solution of ahypochlorite such as sodium hypochlorite or the like, for example, afterthe above-mentioned washing is performed. In this case, decomposition,removal and rinse may be performed normally with an aqueous solution ofa 100-80,000 ppm hypochlorite at temperatures of 5-95 degrees C for1-500 hours. It is also effective to make the resulting PEEK filtrationmembrane sufficiently hydrophilic in advance by immersing it in anaqueous organic solution such as ethanol or the like.

After step (D), it is preferred to preserve the PEEK membrane from beingdried, i.e., under the condition of immersing the membrane in water, analcohol, an aqueous solution of an alcohol, formalin or a mixturethereof, or under the condition of impregnating an aqueous solventthereof into the inside and the surface of the membrane. A preferred wayof preservation is to immerse the membrane in formalin or, to immerse itin or to impregnate it with an alcohol such as ethylene glycol,diethylene glycol, triethylene glycol, tetraethylene glycol, propyleneglycol, dipropylene glycol, tripropylene glycol, low molecular weightpolyethylene glycol, glycerol or the like or a mixture thereof. If thePEEK membrane is dried after step (D), its water permeability tends tobe lowered, which is not preferred.

In accordance with the present invention, it is preferred to subject themembrane to heat treatment after the above-mentioned rinse step by usinga heat stabilizing solvent under a wet condition at temperatures betweenthe glass transition and melting points of PEEK. The temperature of theheat treatment ranges preferably from the glass transition point plus 20degrees C to the melting point minus 20 degrees C of PEEK and morepreferably from the glass transition point plus 50 degrees C to themelting point minus 50 degrees C. For example, if PEEK is composed ofthe repeating units represented by formula (1) of Group 1, thetemperature of the heat treatment ranges preferably from 150 to 320degrees C and more preferably from 200 to 280 degrees C. The glasstransition and melting points of PEEK mentioned in the present inventionmean those obtained by measuring with a differential thermal analyzerwhile raising the temperature at a rate of 10 degrees C/min.

In accordance with the instant invention, the PEEK membrane ismaintained under the wet condition during the heat treatment by use of aheat stabilizing solvent. The heat stabilizing solvents mentioned in thepresent invention mean those that are used for the heat treatment toincrease the crystallinity of the PEEK membrane and maintain themembrane in the wet condition during the treatment. Further, the wetcondition mentioned in the present invention refers to the conditionwherein the said membrane is wet with the heat stabilizing solvent usedfor the heat treatment. In more detail, it refers to the conditionwherein the membrane is wet from its surface to the inside with the heatstabilizing solvent or the like that may bring about the wet conditionor the condition wherein the membrane is wholly immersed in the saidsolvent. The condition is attained by applying or atomizing the solventto the surface of the PEEK membrane, impregnating the solvent into themembrane or immersing the membrane in the said solvent. For example, adesirable wet condition according to the instant invention is thecondition wherein the said solvent is uniformly applied to orimpregnated into the PEEK membrane 0.5 time or more, preferably 2 timesor more, more preferably 10 times or more based on the weight of thePEEK membrane, although depending upon the porosity of the PEEK membraneand the specific gravity of the solvent used for obtaining the wetcondition.

The heat stabilizing solvents used for the heat treatment of the presentinvention are not limited to specific ones if they are solvents that donot dissolve the PEEK membrane and that are stable during the saidtreatment. Of them, the solvents with solubility parameters of 7 to 17are used preferably. The solvents having solubility parameters rangingmore preferably from 7 to 15, particularly preferably from 8 to 13 areused. If a heat stabilizing solvent outside the above ranges is used,the water permeability of the resulting PEEK membrane is remarkablylowered.

The solubility parameter according to the instant invention is denotedby the following formula:

    solubility parameter=(ΔE.sub.v /V).sup.1/2

wherein ΔE_(v) is molar evaporation energy, nearly equal to ΔH-RTwherein ΔH is vapor heat, R is the gas constant and T is absolutetemperature, and V is the molar volume of a solvent.

Solubility parameters are described in many scientific literatures andbooks. Particularly, "Polymer Data Handbook, Basic Edition", compiled byThe Society of Polymer Science, Japan and published by Baifukan Co.,Ltd. has tables on solubility parameters by solvent, so that a decisionmay be made on the choice of the heat stabilizing solvents suitable forthe instant invention.

Other literatures giving considerations to solubility parameters includeInd. Chem. Prod. Res. Dev. 8, Mar. 1969, p.2-11, Chemical Reviews,75(1975), p.731-753, and Encyclopedia of Chemical Technology, 2ndEdition, Supplement Volume (1971), p.889-910.

Examples of the heat stabilizing solvents used according to the presentinvention include alcohols such as methanol, ethanol, n-propanol,n-butanol, isobutanol, sec-butyl alcohol, t-butyl alcohol, n-pentanol,n-hexanol, 2-ethylbutanol, n-octanol, ethyl hexanol, 1-dodecanol,3,5,5,-trimethyl hexanol, cylohexanol, methyl isobutylcarbinol, n-amylalcohol, allyl alcohol, lauryl alcohol, benzyl alcohol, furfurylalcohol, n-heptanol, 1,4-butanediol, 1,3-butanediol, 2,3-butanediol,ethylene glycol, diethylene glycol, triethylene glycol, tetraethyleneglycol, propylene glycol, dipropylene glycol, tripropylene glycol,tetraethylene glycol, neophenyl glycol, 1,5-pentanediol,2,4-pentanediol, 2,5-pentanediol, glycerol, polyethylene glycol,polypropylene glycol and the like, ethers such as dimethyl ether,diethyl ether, ethyl methyl ether, isopropyl ether, dipropyl ether,diisopropyl ether, butyl ether, dibenzyl ether, dihexyl ether, diamylether, ethyl isobutyl ether, methylisobutyl ether, diacetone alcoholmethyl ether, dichloroethyl ether, diphenyl ether, ethyleneglycolmonomethyl ether, diethyleneglycol monomethyl ether, triethyleneglycolmonomethyl ether, tetraethyleneglycol monomethyl ether, propyleneglycolmonomethyl ether, ethyleneglycol dimethyl ether, diethyleneglycoldimethyl ether, triethyleneglycol dimethyl ether, tetraethyleneglycoldimethyl ether, propyleneglycol dimethyl ether, ethyleneglycol monoethylether, diethyleneglycol monoethyl ether, triethyleneglycol monoethylether, tetraethylene glycol monomethyl ether, propyleneglycol monoethylether, ethyleneglycol diethyl ether, diethyleneglycol dimethyl ether,triethyleneglycol diethyl ether, tetraethyleneglycol dimethyl ether,propyleneglycol diethyl ether, ethyleneglycol monoisopropyl ether,diethyleneglycol monoisopropyl ether, triethyleneglycol monoisopropylether, tetraethyleneglycol monoisopropyl ether, propyleneglycolmonoisopropyl ether, ethyleneglycol diisopropyl ether, diethyleneglycoldiisopropyl ether, triethyleneglycol diisopropyl ether,tetraethyleneglycol diisopropyl ether, propyleneglycol diisopropylether, ethyleneglycol monophenyl ether, diethyleneglycol monophenylether, triethyleneglycol monophenyl ether, tetraethyleneglycolmonophenyl ether, propyleneglycol monophenyl ether, ethyleneglycoldiphenyl ether, diethyleneglycol diphenyl ether, triethyleneglycoldiphenyl ether, tetraethyleneglycol diphenyl ether, propyleneglycoldiphenyl ether, methyl-2-pentanediol-1,3-methyl-2-pentanediol-2,4,ethylhexanediol-1,3 and the like, acetals such as 1,4-dioxane, furan,furfural, tetrahydrofuran and the like, esters such as methyl acetate,ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate,isobutyl acetate, sec-butyl acetate, n-amyl acetate, isoamyl acetate,sec-amyl acetate, vinyl acetate, allyl acetate, methylamyl acetate,butyl stearate, methyl formate, ethyl formate, propyl formate, n-butylformate, isobutyl formate, n-amyl formate, isoamyl formate, methylbutyrate, ethyl butyrate, isobutyl butyrate, n-butyl butyrate, propylbutyrate, isopropyl isobutyrate, methyl propionate, ethyl propionate,butyl propionate, propyl propionate, ethyl lactate, methyl lactate,n-butyl lactate, methyl benzoate, ethyl benzoate, ethyl acrylate, methylacrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate,methyl isobutyrate, ethyl isobutyrate, dimethyl phthalate, diethylphthalate, dipropyl phthalate, dibutyl phthalate, dimethyl oxalate,diethyl oxalate, dimethyl sebacate, diethyl sebacate, dibutyl sebacate,dioctyl sebacate, butyrolactone, caprolactone, methyl caprolactone,propiolactone, dioctyl phthalate and the like, ketones such as acetone,methyl ethyl ketone, methyl propyl kentone, methyl n-butyl ketone,methyl amyl ketone, diethyl ketone, diisobutyl ketone, diisopropylketone, methyl isoamyl ketone, ethyl amyl ketone, methyl isobutylketone, methyl isopropyl ketone, methyl nonyl ketone, cyclopentanone,cyclobutanedione, methylcyclohexanone, acetophenone, diacetone alcohol,mesityl oxide, acrolein, benzophenone, chlorobenzophenone,dichlorobenzophenone, difluorobenzophenone, fluorobenzophenone,hydroxybenzophenone, dihydroxybenzophenone, difluoroterephthalophenone,dichloroterephthalophenone, dihydroxyterephthalophenone and the like,aldehydes such as acetaldehyde, benzaldehyde, butylaldehyde and thelike, hydrocarbons such as hexane, heptane, octane, cyclohexane, decane,methyl cyclohexane, tetrahydronaphthalene, benzene, toluene, xylene,styrene, ethyl benzene, n-propyl benzene, cyclopentane and the like,halogenated hydrocarbons such as methyl chloride, methylene chloride,trichloromethane, carbon tetrachloride, ethyl chloride, ethylidenechloride, methyl iodide, ethyl iodide, benzene iodide, bromobenzene,chlorobenzene, dichlorobenzene, trichlorobenzene and the like, fattyacids and phenols such as formic acid, acetic acid, butyric acid, maleicacid, propionic acid, acetic anhydride, propionic anhydride, succinicanhydride, maleic anhydride, acrylic acid, methacrylic acid, phenol,m-cresol, bisphenol A and the like, nitrogen compounds such asnitromethane, nitroethane, nitropropane, nitrooctane, nitrobenzene,methylamine, ethylamine, diethylamine, triethylamine, butylamine,dibutylamine, tributylamine, amylamine, ethylenediamine, N,N-dimethylnitroamine, triethylene tetramine, formamide, N-methyl formamide,N-ethyl formamide, methyl acetamide, N-ethyl acetamide, N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethyl acetamide, tetramethyloxyamide, hexamethyl phosphorylamide, aniline, dimethyl aniline,acetonitrile, chloroacetonitrile, n-butyronitrile, benzonitrile,capronitrile, propionitrile, acrylonitrile, malononitrile,n-valeronitrile, quinoline, morpholine, N-ethyl morpholine, N-acetylmorpholine, N-formyl morpholine, α-pyrrolidone, N-methyl-2-pyrrolidone,pyridine, piperidine, N-acetyl piperidine, N-formyl piperidine, N-acetylpiperidine, N,N-diacetyl piperazine, hydrazine, phenyl hydrazine,ε-caprolactam and the like, carbonates such as dimethyl carbonate,diethyl carbonate, methyl ethyl carbonate, diphenyl carbonate, methylphenyl carbonate, ethyl phenyl carbonate and the like, sulfur compoundssuch as methyl ethyl sulfone, tetramethylene sulfone, dimethyl sulfide,carbon disulfide, methyl tetramethylene sulfone, methyl propyl sulfone,dimethyl sulfone, dimethyl sulfoxide, dimethyl tetramethylene sulfone,diethyl sulfone, sulfolane, thiophene, dipropyl sulfone, diphenylsulfone, difluorodiphenyl sulfone, dichlorodiphenyl sulfone,dihydroxydiphenyl sulfone and the like, phosphorus compounds such asdibutylphenyl phosphate, tricresyl phosphate, triphenyl phosphite,hexamethylphosphoric triamide and the like, and mixtures thereof.

Of the aforesaid heat stabilizing solvents, particularly preferred foruse are 1,4-butanediol, 1,3-butanediol, ethylene glycol, diethyleneglycol, triethylene glycol, tetraethylene glycol, propylene glycol,dipropylene glycol, tripropylene glycol, glycerol, polyethylene glycol,polypropylene glycol, N,N-dimethyl formamide, N,N-dimethyl acetamide,N-methyl-2-pyrrolidone and mixtures thereof. If polyethylene glycol orpolypropylene glycol is used as a heat stabilizing solvent according tothe instant invention, each weight average molecular weight is notparticularly limited but ranges preferably from 100 to 2,000.

The time necessary for the heat treatment of the instant invention isnot especially restricted but is in the range of preferably 30 secondsto 100 hours, more preferably 1 minute to 50 hours, still morepreferably 1 minute to 2 hours.

If the PEEK membrane is under the wet condition of being immersed in aheat stabilizing solvent, the heat treatment of the present inventionmay be carried out in an open system or in a closed system wherein anautoclave or the like is used, while if the PEEK membrane is in the wetcondition of applying a heat stabilizing solvent thereto or impregnatingthe solvent thereinto, the heat treatment may be performed in an opensystem by use of an oven or the like or in a closed system by use of anautoclave or the like. However, in the open system, the PEEK membrane isin some cases deformed or damaged on its surface because of the intenseevaporation of the heat stabilizing solvent during the heat treatment.Therefore, in the open system in which an oven or the like is used,attention must be paid so that the PEEK membrane may not deviate fromthe wet condition owing to the evaporation of the heat stabilizingsolvent.

Furthermore, when the heat treatment of the present invention isperformed, the atmosphere of the heat stabilizing solvent may be the airor an inert gas such as nitrogen, argon, helium or the like. If oxygenin the air has such an influence that it degrades the water permeabilityor fractionating characteristics of the resulting PEEK filtrationmembrane because oxygen deteriorates the heat stabilizing solvent duringthe heat treatment, it is preferred that the heat treatment is carriedout under an inert gas.

If the heat treatment follows step (D) consecutively, it is effectivethat residual low boiling point solvents such as coagulation liquids,rinse solvents, etc. are removed from the PEEK filtration membrane priorto the said treatment. Further, it is preferred that the low boilingpoint solvents remaining in the PEEK filtration membrane after step (D)are substituted by a portion of the heat stabilizing solvent to be usedin the heat treatment. If the said low boiling point solvents remain inthe surface or inner regions of the membrane at the time of the heattreatment, it is undesirable that the semipermeable membrane is deformedor damaged in some cases on account of the boiling of the residual lowboiling point solvents.

Furthermore, if the heat treatment is performed consecutively followingstep (D), the PEEK filtration membrane may be impregnated with a highmolecular weight compound as treatment prior to the heat treatmentaccording to necessity. This tends to result in a smaller differencebetween the water permeability of the said membrane before and after theheat treatment.

Any of the said high molecular weight compounds may be used withoutparticular restriction if it does not decompose at temperatures notlower than the glass transition point of PEEK and particularly at thetemperatures of the heat treatment to be performed thereafter, has nochemical influence on PEEK and further can be removed from the PEEKfiltration membrane with a solvent after the heat treatment.

If the heat treatment is carried out according to the instant invention,the residual heat stabilizing solvent used for the heat treatment isnormally removed by rinsing after the heat treatment. As the said rinsesolvent, water or an organic solvent such as an alcohol, acetone or thelike is used preferably and the temperature for rinsing is in the rangebetween 5 and 120 degrees C. However, it is undesirable that thedeformation of the PEEK membrane and the damage of its surface tend totake place if the rinse solvent for removing the residual heatstabilizing solvent boils under reflux or the like. The rinse step forwashing away the residual heat stabilizing solvent may be omitted, ifthere is no problem with the solvent in using the PEEK membrane as aseparation membrane.

If the PEEK membrane dries in its inner regions and on its surface evenafter the heat treatment of the present invention, the aforesaidpreservation method may be effectively applied thereto because the waterpermeability tends to be lowered.

The PEEK membrane of the present invention has an ion exchange capacityof 0 to 0.5 meq/g. Especially, in order to enhance its heat and chemicalresistance, the ion exchange capacity should range preferably from 0.005to 0.3 meq/g and more preferably from 0.005 to 0.2 meq/g. To enhance itshydrophilic property and fouling resistance, it is preferred that theion exchange capacity is in the range of 0.1 to 0.5 meq/g.

The aforesaid heat treatment enhances the crystallinity of the PEEKmembrane prepared according to the instant invention and as a result,may further improve the heat and chemical resistance thereof. If theheat and chemical resistance are required at a temperature exceeding 100degrees C, the crystallinity is preferably at least 10 percent byweight, more preferably at least 20 percent by weight, and still morepreferably at least 25 percent by weight, though depending upon theapplication field of the PEEK membrane. The crystallinity of the PEEKmembrane in the present invention is expressed by the ratio of thecrystallized component to the total of the PEEK membrane on a weightbasis and is measured according to the wide-angle X-ray diffractometryreported by Blundell and Osborn (Polymer, 24, 953, 1983).

The PEEK membrane obtained according to the present invention isexcellent in chemical resistance despite being manufactured by usingsulfuric acid and, for example, can be used in an acid or alkalinesolvent or even in a polar organic solvent. If the crystallinity of thePEEK membrane is increased by the aforesaid heat treatment, not only isthe chemical resistance further improved but also the mechanicalstrength, for example, tensile strength and bursting strength areenhanced remarkably. Furthermore, regarding heat resistance, the PEEKmembrane thus treated has excellent heat stability in water of 100degrees C and even in water having temperatures exceeding 150 degrees C.

Because membrane formation is performed with the use of concentratedsulfuric acid highly capable of dissolving PEEK, it is possible tocontrol the fractionating and water permeability characteristicsthroughout a wide range and to prepare both an asymmetric membranehaving the surface and inner regions with differing pore sizes and asymmetric membrane having pores uniform in size throughout the wholethereof, depending upon the composition of the membrane forming stocksolution and the coagulation conditions thereof.

The PEEK membrane obtained according to the present invention maycontain a hygroscopic polymer such as polyvinyl pyrrolidone or the likeor further crosslink therewith to such an extent that it has noinfluence on the properties of the PEEK membrane such as heat resistanceand the like.

The form of the PEEK membrane prepared according to the presentinvention is not especially restricted but is preferably that of ahollow fiber. In this case, the wall thickness of the hollow fiber isnot particularly restricted but ranges generally from 5 to 3,000 μm andparticularly preferably from 10 to 1,000 μm. The outside diameter of thesaid hollow fiber is not especially limited, but ranges generally from10 to 10,000 μm and particularly preferably from 300 to 5,000 μm.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinbelow, the present invention will be described in more detail withreference to the following examples, but they should not be construed tolimit the scope of the present invention.

First, descriptions will be given on the measuring methods to be adoptedin the following examples and comparative examples.

(1) Method of measuring ion exchange capacity

Five grams of sodium chloride is dissolved in distilled water so as toobtain 100 ml of an aqueous sodium chloride solution, in which 0.1 g ofa PEEK membrane is immersed. After agitation for two hours at roomtemperature, the membrane is taken out therefrom and the said sodiumchloride solution is titrated with a 0.025 N aqueous sodium hydroxidesolution. On the other hand, the membrane thus taken out is immersed in50 ml of 0.1 N sulfuric acid and allowed to stand for two hours at roomtemperature. The said membrane is taken out from the sulfuric acid andis dried under vacuum at 50 degrees C for ca. 24 hours after beingwashed with pure water until it is neutralized in order to measure theweight of the membrane. Based upon the titer and the weight of themembrane obtained in the above-mentioned method, the ion exchangecapacity of the PEEK membrane is calculated according to the followingformula:

    ion exchange capacity (meq/g)=titer (ml)×0.025÷membrane weight (g)

Provided that the ion exchange capacity of the membrane is less than 0.1meq/g, the measurement is made by increasing the weight of the membraneor the sodium ion absorbed in the above-mentioned method is measured bymeans of absorption spectroscopy.

(2) Water permeability

If the form of the membrane is that of a flat-sheet membrane, theflat-sheet membrane is placed in a plastic filter holder (Trade name:PP-25, a product of Advantec Toyo, Co., Ltd.) and 25 degree C distilledwater is passed through the membrane at a pressure of 1 kg/cm² from theside exposed to a coagulation liquid at the time of membrane formationto measure the volume of the distilled water passed therethrough for 20minutes. Water permeability is expressed as flux by calculating the saidvolume in terms of l/m² ·hr·kg/cm². Further, if the form of the membraneis that of a hollow fiber, 25 degree C distilled water is injected at apressure of 1 kg/cm² into the bore of a 30-cm long hollow fiber whoseone end is sealed to measure the volume of the distilled water permeatedthrough the wall of the hollow fiber for 20 minutes. Water permeabilityis expressed as flux by calculating the said volume in terms of l/m²·hr·kg/cm².

(3) Method of measuring the hollow fiber membrane rejection ratio ofdextran molecules

Dextran (a product of Pharmacia) having a weight average molecularweight of 10,000 is added to distilled water to prepare a feed dextransolution so that it may have a concentration of 0.5 percent by weight.The said feed solution is injected through the bore of a hollow fibermembrane having a length of 30 cm at a temperature of 25 degrees C, alinear rate of 1 m/sec. and an average filtration pressure of 1 kg/cm²for 10 minutes to obtain a filtrate through the wall of the hollow fibermembrane. The rejection ratio of the hollow fiber membrane is obtainedfrom the following formula:

    rejection ratio (%)=(1-C/C.sub.0)×100

wherein C and C₀ indicate the respective concentrations of the obtainedfiltrate and the feed dextran solution.

(4) Method of measuring the hollow fiber membrane rejection ratio ofpolystyrene latex particles

Polystyrene latex (a product of Seradyn, Inc.) having a particle size of0.212 μm is added to distilled water to prepare a feed suspension sothat it may have a concentration of 200 ppm. The rejection ratio isobtained according to the same filtration and calculation methods as inthe aforesaid dextran case.

(5) Method of measuring the crystallinity of hollow fiber membrane

By use of X-ray diffraction equipment (MXP-18, a product of MAC ScienceCo.,Ltd.), the X-ray obtained from a Cu target with an accelerationvoltage of 50 kV and acceleration current of 200 mA is made monochromicwith an Ni monochrometer. Specimens are placed on a fiber sample tableand subjected to measurement by means of penetration. The scatteredX-ray from each specimen is scanned in the range of 12° to 32° and 50points are adopted per 1° and the measurement is performed for 1.2seconds per point.

EXAMPLE 1

Ten grams of PEEK comprising the repeating units represented by Formula(1) of Group 1 (a product of ICI, VICTREX PEEK 450G) ground intoparticles of ca. 0.3 mm in diameter was added to 90 grams of 97.5%concentrated sulfuric acid (a product of Kanto Chemical Co., Inc.) at 10degrees C and was uniformly dissolved therein in a closed system whilethe temperature was being kept at 10 degrees C to prepare a membraneforming stock solution. The stock solution was deaerated under vacuumwhile the temperature was being kept at 10 degrees C. The time requiredfor the dissolution and deaeration totaled 4 hours.

The membrane forming stock solution was cooled to 5 degrees Cimmediately after the deaeration and was allowed to be left for 40hours.

The stock solution thus prepared was applied to a glass plate to providea thickness of 100 μm on the glass plate and was consecutively immersedand coagulated in 23 degree C distilled water, in which it was allowedto stand for 5 minutes. The resulting membrane was thereafter taken outfrom the water, immersed in running water for 1 hour and soaked inethanol for 2 hours to wash away the residual sulfuric acid remaining inthe membrane.

The PEEK flat-sheet membrane thus obtained had an ion exchange capacityof 0.07 meq/g and was not sulfonated substantially. The flux of themembrane was 170 l/m² ·hr·kg/cm² Further, as a result of observing thesurface and cross-section of the membrane at a magnification of 10,000times with a scanning electron microscope, it was found that themembrane was asymmetrical in structure, having a surface with no openpores, a tight skin layer in the neighborhood of the surface andinternal voids which became larger gradually as they went inwards.

EXAMPLE 2

Ten grams of PEEK comprising the repeating units represented by Formula(1) of Group 1 (a product of ICI, VICTREX PEEK 450G) ground intoparticles of ca. 1 mm in diameter was added to 90 grams of 94.5%concentrated sulfuric acid (a product of Kanto Chemical Co., Inc.) at 18degrees C and was uniformly dissolved therein in a closed system whilethe temperature was being kept at 10 degrees C to prepare a membraneforming stock solution. The stock solution was deaerated under vacuumwhile the temperature was being kept at 10 degrees C. The time requiredfor the dissolution and deaeration totaled 5 hours.

The membrane forming stock solution was cooled to 5 degrees Cimmediately after the deaeration and was allowed to stand for 50 hours.

The stock solution thus prepared was applied to a glass plate to providea thickness of 100 μm on the glass plate and was consecutively immersedand coagulated in 60% sulfuric acid of 23 degrees C, in which it wasallowed to be left for 5 minutes. The resulting membrane was thereaftertaken out from the sulfuric acid, immersed in running water for 1 hourand soaked in ethanol for 2 hours to wash away the sulfuric acidremaining in the membrane.

The PEEK flat-sheet membrane thus obtained had an ion exchange capacityof 0.04 meq/g and was not sulfonated substantially. The flux of themembrane was 350 l/m² ·hr·kg/cm². Further, as a result of observing thesurface and cross-section of the membrane at a magnification of 10,000times with a scanning electron microscope, it was found that themembrane was asymmetrical in structure, having a surface with no openpores, a tight skin layer in the vicinity of the surface and internalvoids which became larger gradually as they went inwards.

EXAMPLE 3

Ten grams of PEEK comprising the repeating units represented by Formula(1) of Group 1 (a product of ICI, VICTREX PEEK 450G) ground intoparticles of ca. 0.3 mm in diameter was added to 90 grams of 97.5%concentrated sulfuric acid (a product of Kanto Chemical Co., Inc.) at 6degrees C, and 3 grams of polyvinyl pyrrolidone (weight averagemolecular weight: 10,000, a product of Kishida Chemical Co., Ltd.) wasfurther added thereto. They were uniformly dissolved therein in a closedsystem while the temperature was being kept at 6 degrees C to prepare amembrane forming stock solution. The stock solution was deaerated undervacuum while the temperature was being kept at 6 degrees C. The timerequired for the dissolution and deaeration totaled 10 hours.

The membrane forming stock solution was cooled to 3 degrees Cimmediately after the deaeration and was allowed to stand for 20 days.

The stock solution thus prepared was applied to a glass plate to providea thickness of 100 μm thereon and immediately immersed and coagulated in60% sulfuric acid of 23 degrees C, in which it was allowed to be leftfor 10 minutes therein. The resulting membrane was thereafter taken outfrom the sulfuric acid, immersed in running water for 1 hour and soakedin ethanol for 2 hours to wash away the sulfuric acid remaining in themembrane. Further, by immersing the membrane in a 25° C. aqueoussolution of 3,000 ppm sodium hypochlorite for 20 hours, residualpolyvinyl pyrrolidone was removed therefrom.

The PEEK flat-sheet membrane thus obtained had an ion exchange capacityof 0.04 meq/g and was not sulfonated substantially. The flux of themembrane was 980 l/m² ·hr·kg/cm². Further, as a result of observing thesurface and cross-section of the membrane at a magnification of 10,000times with a scanning electron microscope, it was found that themembrane was asymmetrical in structure, having a surface with no openpores, a tight skin layer in the neighborhood of the surface andinternal voids which became larger gradually as they went inwards.

COMPARATIVE EXAMPLE 1

Ten grams of PEEK comprising the repeating units represented by Formula(1) of Group 1 (a product of ICI, VICTREX PEEK 450G, particle size: ca.4 mm) was added to 90 grams of 97.5% concentrated sulfuric acid (aproduct of Kanto Chemical Co., Inc.) at 25 degrees C and was uniformlydissolved therein in a closed system while the temperature was beingkept at 25 degrees C to prepare a membrane forming stock solution. Thestock solution was deaerated under vacuum while the temperature wasbeing kept at 25 degrees C. The time required for the dissolution anddeaeration totaled 4 hours.

After the deaeration, the membrane forming stock solution was allowed tostand for 20 hours, while it was being kept at a temperature of 25degrees C.

The stock solution thus prepared was applied to a glass plate to providea thickness of 100 μm on the glass plate and immediately immersed andcoagulated in 23 degree C distilled water, in which it was allowed to beleft for 5 minutes. The resulting membrane was thereafter taken out fromthe water, immersed in running water for 1 hour and soaked in ethanolfor 2 hours to wash away the sulfuric acid remaining in the membrane.

The membrane thus obtained swelled when immersed in ethanol. Further,the said membrane was swelled and intensively deformed in boiling water.

The PEEK flat-sheet membrane obtained in this comparative example had anion exchange capacity of 0.7 meq/g and was sulfonated substantially.

COMPARATIVE EXAMPLE 2

Ten grams of PEEK comprising the repeating units represented by Formula(1) of Group 1 (a product of ICI, VICTREX PEEK 450G, particle size: ca.4 mm) was added to 90 grams of 97.5% concentrated sulfuric acid (aproduct of Kanto Chemical Co., Inc.) at 25 degrees C and was uniformlydissolved therein in a closed system while the temperature was beingkept at 25 degrees C to prepare a membrane forming stock solution. Thestock solution was deaerated under vacuum while the temperature wasbeing kept at 25 degrees C. The time required for the dissolution anddeaeration totaled 4 hours.

After the deaeration, the membrane forming stock solution was allowed tostand for 20 days while the stock solution was being kept at 18 degreesC.

The stock solution thus prepared was applied to a glass plate to providea thickness of 100 μm on the glass plate and was immediately immersedand coagulated in 23 degree C distilled water, in which it was allowedto be left therein for 5 minutes. The resulting membrane was then takenout from the water and immersed in running water for 1 hour.

The obtained membrane had an ion exchange capacity of 1.4 meq/g and wassulfonated substantially. As soon as the said membrane was immersed andwashed in ethanol, it dissolved therein.

EXAMPLE 4

One hundred grams of PEEK comprising the repeating units represented byFormula (1) of Group 1 (a product of ICI, VICTREX PEEK 450G) ground intoparticles of ca. 0.3 mm in diameter was added to 900 grams of 97.5%concentrated sulfuric acid (a product of Kanto Chemical Co., Inc.) at 10degrees C and was uniformly dissolved therein in a closed system whilethe temperature was being kept at 10 degrees C to prepare a membraneforming stock solution. The stock solution was deaerated under vacuumwhile the temperature was being kept at 10 degrees C. The time requiredfor the dissolution and deaeration totaled 10 hours.

Immediately after the deaeration, the membrane forming stock solutionwas cooled to 6 degrees C.

The stock solution, of which the temperature was kept at 6 degrees C,was extruded through a tube-in-orifice type spinneret for hollow fiberspinning at a flow rate of 6 ml/min., while 25° C. water was beinginjected as a coagulant through the bore at the same time. The solutionthus extruded was immersed in a 24° C. water bath underlaid 9 cm belowthe spinneret and was wound after coagulation. About 80 minutes wasrequired for the spinning from the beginning to the end.

The wound hollow fiber membrane was washed by immersing it in 25° C.running water for 10 hours and then in 25° C. ethanol for 10 hours.

The portion of the hollow fiber membrane wound immediately after thebeginning of the spinning had an ion exchange capacity of 0.02 meq/g,while the other portion of the hollow fiber membrane wound in theneighborhood of the end of the spinning had also an ion exchangecapacity of 0.02 meq/g. Thus, the PEEK hollow fiber membrane could beobtained by spinning without substantial sulfonation.

The resulting hollow fiber membrane was heat treated, without beingdried, in 90 degree C triethylene glycol for 1 hour and further in 200degree C triethylene glycol for 1 hour. After this treatment, the PEEKhollow fiber membrane was immersed in 25 degree C ethanol to removeresidual triethylene glycol therefrom and was kept in water.

The PEEK hollow fiber membrane thus obtained had a flux of 48 l/m²·hr·kg/cm², crystallinity of 24 percent measured by the wide-angle X-raydiffraction analysis and a dextran rejection ratio of 92 percent.

The said membrane was excellent in heat resistance without showing anychange in appearance and water permeability even if immersed in 130degree C water for 10 hours.

EXAMPLE 5

One hundred grams of PEEK comprising the repeating units represented byFormula (1) of Group 1 (a product of ICI, VICTREX PEEK 450G) ground intoparticles of ca. 0.3 mm in diameter was added to 900 grams of 97.5%concentrated sulfuric acid (a product of Kanto Chemical Co., Inc.) at 8degrees C, and 30 grams of polyvinyl pyrrolidone (weight averagemolecular weight: 10,000, a product of Kishida Chemical Co., Ltd.) wasfurther added thereto. They were uniformly dissolved therein in a closedsystem while the temperature was being kept at 8 degrees C to prepare amembrane forming stock solution. The stock solution was deaerated undervacuum while the temperature was being kept at 8 degrees C. The timerequired for the dissolution and deaeration totaled 15 hours.

Immediately after the deaeration, the membrane forming stock solutionwas cooled to 3 degrees C.

The membrane forming stock solution, of which the temperature was keptat 3 degrees C, was extruded through a tube-in-orifice type spinneret ata flow rate of 6 ml/min., while 25° C. water was being injected as aninside coagulant into the bore at the same time. The stock solution thusextruded was immersed in a 24° C. water bath underlaid 9 cm below thespinneret and was wound after it coagulated. About 90 minutes wasrequired for spinning from the beginning to the end.

The wound hollow fiber membrane was washed by immersing it in 25° C.running water for 10 hours and then in 25° C. ethanol for 10 hours. Itwas further rinsed by immersing it in a 26° C. aqueous solution ofsodium hypochorite having a concentration of 3,000 ppm for 20 hours.

The portion of the hollow fiber membrane wound immediately after thebeginning of the spinning had an ion exchange capacity of 0.01 meq/g,while the other portion of the hollow fiber membrane wound near the endof the spinning also had an ion exchange capacity of 0.01 meq/g. Thus,the PEEK hollow fiber membrane could be spun without being sulfonatedsubstantially.

The obtained hollow fiber membrane, without being dried, was heattreated in 90° C. triethylene glycol for 1 hour and further heat treatedin 200° C. triethylene glycol for 1 hour. After this treatment, the PEEKhollow fiber membrane was immersed in 25° C. ethanol to remove residualtriethylene glycol therefrom and was thereafter kept in water.

The PEEK hollow fiber membrane thus obtained had a flux of 80 l/m²·hr·kg/cm², crystallinity of 24 percent measured by the wide-angle X-raydiffraction analysis and a dextran rejection ratio of 92 percent.

Measured under the condition of a 5 cm distance between grips, 50mm/min. grip separation speed and 25 degrees C, the tensile strength ofthe PEEK hollow fiber membrane was found to be 68 kg/cm².

EXAMPLE 6

One hundred grams of PEEK comprising the repeating units represented byFormula (1) of Group 1 (a product of ICI, VICTREX PEEK 450G) ground intoparticles of ca. 0.3 mm in diameter was added to 900 grams of 97.5%concentrated sulfuric acid (a product of Kanto Chemical Co., Inc.) at 10degrees C, and 30 grams of polyvinyl pyrrolidone (weight averagemolecular weight: 10,000, a product of Kishida Chemical Co., Ltd.) wasfurther added thereto. They were uniformly dissolved therein in a closedsystem while keeping the temperature at 10 degrees C to prepare amembrane forming stock solution. The stock solution was deaerated undervacuum while keeping the temperature at 10 degrees C. The time requiredfor the dissolution and deaeration totaled 10 hours.

Immediately after the deaeration, the membrane forming stock solutionwas cooled to 3 degrees C.

The stock solution, of which the temperature was kept at 3 degrees C,was extruded through a tube-in-orifice type spinneret at a flow rate of6 ml/min., while passing 25° C. 50% sulfuric acid as an inside coagulantthrough the bore. The stock solution thus extruded was immersed in a 24°C. water bath underlaid 5 cm below the spinneret and was wound after itcoagulated. About 90 minutes was required for spinning from thebeginning to the end.

The wound hollow fiber membrane was washed by immersing it in 25° C.running water for 10 hours and further immersing in 25° C. ethanol for10 hours. The membrane was thereafter rinsed by immersing it in a 26° C.aqueous solution of sodium hypochorite having a concentration of 3,000ppm for 20 hours.

The portion of the hollow fiber membrane wound immediately after thebeginning of the spinning had an ion exchange capacity of 0.02 meq/g,while the other portion of the hollow fiber membrane wound near the endof the spinning also had an ion exchange capacity of 0.02 meq/g.Therefore, the PEEK hollow fiber membrane could be spun without beingsulfonated substantially.

The PEEK hollow fiber membrane thus obtained had a flux of 240 l/m²·hr·kg/cm² and a dextran rejection ratio of 72 percent.

COMPARATIVE EXAMPLE 3

The membrane forming stock solution prepared and kept in the same manneras in Comparative example 1 was spun into a hollow fiber membrane in thesame way as in Example 6.

The obtained hollow fiber membrane had an ion exchange capacity of 0.84meq/g. Its tensile strength was measured as in Example 5 and resulted ina value of 21 kg/cm². The membrane was clearly inferior in tensilestrength to that obtained in Example 5.

Industrial Applicability

In accordance with the present invention it is possible toadvantageously obtain a polyether ether ketone membrane by usingsulfuric acid which is easy to handle and available at low pricesgenerally from a commercial viewpoint, instead of using organic acids orhydrofluoric acid in its preparation. The PEEK membrane obtained by theproduction process of the present invention is excellent in mechanicalstrength and heat and chemical resistance and has low elutioncharacteristics. Further, the use of sulfuric acid which is highlyeffective in dissolving PEEK, makes it possible to control membraneperformance throughout a wide range, so that a membrane having anexcellent balance of water permeability and fractionatingcharacteristics can be obtained.

For the above reasons, the PEEK membrane of the present invention may beadvantageously employed for filtration in the preparation of ultrapurewater for semiconductors in the electronic industry and for filtrationin the fields of medical appliances, pharmaceuticals and foods. Further,the PEEK membrane of the present invention may also be advantageouslyemployed for severe use in stable filtration for a long term at hightemperatures exceeding 100 degrees C, if the crystallinity of themembrane is heightened by the heat treatment wherein a heat stabilizingsolvent is used.

What is claimed is:
 1. In a process for preparing a polyether etherketone membrane which comprises dissolving polyether ether ketone in asolvent to obtain a membrane forming stock solution, forming thesolution into a predetermined shape and bringing the formed solutioninto contact with a coagulation liquid capable of coagulating saidketone, the improvement which comprises(a) using concentrated sulfuricacid as solvent; (b) after dissolving said ketone in the solvent,maintaining the resulting stock solution at 15 degrees C or lower; andthen (c) contacting the stock solution with the coagulation liquid,whereupon initiation of the membrane formation occurs, said polyetherether ketone membrane having an ion exchange capacity of 0.005 to 0.5meq/g.
 2. The process according to claim 1, wherein the concentration ofconcentrated sulfuric acid is 94 percent or more.
 3. The processaccording to claim 1, wherein the membrane forming stock solutioncontains a thickening agent.
 4. The process according to claim 3,wherein the thickening agent is polyvinyl pyrrolidone or polyethyleneglycol.
 5. The process according to claim 1, wherein said ketone isdissolved in the solvent at a temperature of 15 degrees C or lower. 6.The process according to claim 1, wherein the coagulation liquid iswater or sulfuric acid having a concentration lower than 70 percent. 7.The process according to claim 6, wherein the coagulation liquidcontains a water-soluble organic solvent.
 8. The process according toclaim 1, wherein the coagulation liquid is a water-soluble organicsolvent.
 9. The process according to claim 1, wherein after membraneformation the membrane is subjected to heat treatment by using a heatstabilizing solvent under a wet condition at temperatures between theglass transition point and the melting point of said ketone.
 10. Theprocess according to claim 9, wherein the temperature of the heattreatment ranges from 150 to 320 degrees C.
 11. A polyether ether ketonemembrane obtained by the process of claim
 10. 12. The process accordingto claim 9, wherein the solubility parameter of the heat stabilizingsolvent ranges from 7 to
 17. 13. A polyether ether ketone membraneobtained by the process of claim 1.